K40 Speech Processor Model K40-7

[Handy Andy]

Way too cool...

​

If you have this circuit INSIDE the radio - you're ok to use nearly any type of Noise canceling mic you have.

This does the amp and clipping for you - this also taps the RX line so you may need to rewire the mic element to SHORT OUT the Audio line like other radios did similar to using the Radio Shack Audio 2 to Pin 4 setups in their DIN 5-pin mic's.

Why? Well, if that line is left open your mic handset wire in the cord will pick up a lot of noise - and since this isn't sending power to the handset - the thing will act like a noisy antenna - it's on (Powered) all the time and the receive audio from the detector, ANL and Squelch are all done before this mic is inserted into the line heading to the Audio amp chip so it will send anything it senses - in with your received audio.

You may need to do this mod to protect you ears from the noise that amp will pick up...

I'm just showing you how to use the DPDT switch to silence the Mic element
By giving it a path to ground thru the shield
- quieting the mic element from extra noise and saving your receive..

You're Mic connector FOLLOWING your radio pin out
Would look and work something like this...
​

So, Pin 1 is audio - but instead of being left open when in RX mode, it shorts to GROUND - giving the Dynamic mic element a place to go to.

Now in some cases like your Pro510 - they use a different wiring scheme...

Pin 1 Ground
Pin 2 Audio
Pin 3 TX
Pin 4 RX

As is the older Uniden/Cobra/Galaxy way...

So you may need to swap Pin 2 and Pin 1 on the Mic connector to work right.

​

 

[secret squirrel]

I have a K40-8. Been running it for years, it refuses to die. I had a couple solder joints repaired over the years. It was an E-bay find. About 10 years ago DTB radio did the peak and tune. The speech processor board has its own adjustments. I have been using an ST65DX noise canceling mic from Sparky's. The mic is a copy of RK56 that Sparky's did and he tried to get the mic element copied. Anyway, that's history and the important thing is that the radio sounds great. If I remember right Dave at DTB has the Sam's for the K40 CBs. If you tune it with a noise canceling mic then keep using that mic. Like Handy Andy noted the speech processor board has a mic pre-amp adjustment. When I tried a regular coffin mic is sounded over driven. The mic combination with the K40 speech processor produces a sound that seems like a lot more radio than it is in actuality.

 

[9C1Driver]

That's funny. The K40-8 was the second cb radio I bought. Ran it for years with that odd speech processor mic of theirs. What a great radio.

 

[NZ8N]

K40 made a non-Speech Clipper/Processor microphone in the same mic shell and cord for these later K40 radios like the Model 6, I don't remember if it was dynamic or crystal (the processor mic used a crystal element). I happen to have 2 of the processor mics in my storage unit. Nostalgia. They didn't need to process (clip) mic audio twice.

 

[Elliott Hart]

Way too cool...

View attachment 40074​

If you have this circuit INSIDE the radio - you're ok to use nearly any type of Noise canceling mic you have.

This does the amp and clipping for you - this also taps the RX line so you may need to rewire the mic element to SHORT OUT the Audio line like other radios did similar to using the Radio Shack Audio 2 to Pin 4 setups in their DIN 5-pin mic's.

Why? Well, if that line is left open your mic handset wire in the cord will pick up a lot of noise - and since this isn't sending power to the handset - the thing will act like a noisy antenna - it's on (Powered) all the time and the receive audio from the detector, ANL and Squelch are all done before this mic is inserted into the line heading to the Audio amp chip so it will send anything it senses - in with your received audio.

You may need to do this mod to protect you ears from the noise that amp will pick up...

View attachment 40075
I'm just showing you how to use the DPDT switch to silence the Mic element
By giving it a path to ground thru the shield
- quieting the mic element from extra noise and saving your receive..

You're Mic connector FOLLOWING your radio pin out
Would look and work something like this...
View attachment 40076​

So, Pin 1 is audio - but instead of being left open when in RX mode, it shorts to GROUND - giving the Dynamic mic element a place to go to.

Now in some cases like your Pro510 - they use a different wiring scheme...

Pin 1 Ground
Pin 2 Audio
Pin 3 TX
Pin 4 RX

As is the older Uniden/Cobra/Galaxy way...

So you may need to swap Pin 2 and Pin 1 on the Mic connector to work right.

​

Thanks for the detailed explanation. I need to figure what mic I'm going use. Maybe a road king 56. Any suggestions?

 

[Handy Andy]

Anything but powered...

Just be careful for the money you spend on the fancy case or handset, doesn't always mean you'll have dynamite audio from the element used in it.

It is usually the opposite, the more they spend on making the looks or feel of the mic "heftier" than a comparable brand - the less money they have to invest in a good element for inside it.

Much of the Noise cancelling on a typical mic handset is the more restricted design of it's windscreen and pop filter - and the cradle the element sits in recesses further back in the handset away from the windscreen, taking in audio in a more of a directional pattern and in some ways, lessening the noise.

But that doesn't make the Mic any better, I've had to perform surgery on several types of handsets and their windscreen designs to open up and expose the element to more of the voice of the user and in some - the needed extra diameter of the hole or removal of dampening fabric helps improve the tone response to make it better for the user.

To direct you to one or the other may not help you - I really have to let you decide - for once you get the mic - you can rework some of its' innards to improve and customize it for your own.

 

[Staybolt]

I have one! Those are killer little radios. I'd just leave it alone.